Liquid inks have been used in many types of ink-jet printers of which the major categories are "Drop-On-Demand" ink-jet and "Continuous" ink-jet. For Drop-On-Demand ink-jet, ink is normally stored in a reservoir and delivered by capillary action to a nozzle in the print head of the printer. A means exists to force a single drop of ink out of the nozzle whenever it is needed to print a single spot on the printed medium (for example, paper). For Continuous ink-jet, ink is forced out of the nozzle as a stream, which is broken into regularly spaced droplets by the application of mechanical perturbations. The trajectory of each drop is controlled typically by determining the change on each droplet and subjecting it to electrostatic forces. The drop can be directed either to a catcher for recycling through the main ink system or can be directed to the printed medium to form a printed spot.
Most ink-jet inks are liquids at room temperature. Liquid inks present various difficulties; for example, they do not respond the same to different printing media. Typically, liquid ink on office papers will produce a feathered appearance because it penetrates and spreads into the paper following fiber lines. Liquid inks that are designed for minimum feathering still require time to set, which may limit the rate that printed pages are stacked.
The print quality usually depends on the type of paper used, which also has an effect on the drying time and on waterfastness. Although water-borne inks have been widely used, they exhibit poor waterfastness. Also, in order to prevent the ink from drying in the jet, high concentrations of humectant such as diethylene glycol have been used. This also leads to a long drying (set) time for the print on the medium and poor print quality.
Liquid inks without curable additives typically are not useful on nonporous surfaces, such as metal, glass or plastic, because they are too prone to smearing.
Further, liquid inks are very sensitive to temperature changes which influence the ink viscosity and interfacial tension, which, in turn, influences the ink interaction with the medium.
It is clear from the foregoing that major problems with liquid ink-jet inks are (i) media dependent print quality, (ii) poor reliability, (iii) poor waterfastness, and (iv) a long drying (set) time for the printed ink.
One method of solving several of the above mentioned problems is to use a phase change or hot melt ink. The ink is normally solid at room temperature. When the ink is heated, the ink melts to form a low viscosity fluid which can be ejected as droplets.
Hot-melt ink was originally used by Berry et al. (U.S. Pat. No. 3,653,932, April 1972) in electrostatically controlled continuous ink-jet printing. The ink was comprised of a waxy component which is solid at room temperature. The term "hot melt ink" defines an ink that is in a solid phase at room temperature and in a fluid phase at the operating temperature. Many different types of hot-melt inks have been used.
These inks are also referred to, for example, in U.S. Pat. Nos. 4,490,731, 3,653,932, 3,715,219 and 4,390,369, entitled "Natural Wax-Containing Ink Jet Inks" and its continuation Ser. No. 507,918, filed Jun. 27, 1983; U.S. Pat. No. 4,361,843, entitled "Improved Ink Jet Compositions and Methods"; U.S. Pat. No. 4,400,215, entitled "Improved Ink Jet Formulation for Reduced Start-Up Problems", and its continuation application U.S. Ser. No. 522,837, filed Aug. 12, 1983; U.S. Ser. No. 394,154, filed Jul. 1, 1982, entitled "Stearic Acid Containing Ink Jet Inks", now abandoned, and its continuation application, U.S. Ser. No. 565,124, filed Dec. 23, 1983; U.S. Pat. No. 4,3861,961, entitled "Heterologous Ink Jet Ink Compositions", and its continuation application, Ser. No. 501,074, filed Jun. 5, 1983; U.S. Ser. No. 668,095, filed Nov. 5, 1984, now abandoned, and its continuation Ser. No. 006,727, filed Jan. 23, 1987; U.S. Ser. No. 672,587, filed Nov. 16, 1984, entitled "Low Corrosion Impulse Ink Jet Ink", now abandoned, and its continuation U.S. Ser. No. 037,062, filed Apr. 13, 1987.
These "hot-melt" inks normally comprise vehicles, such as natural waxes, resins and/or long chain fatty acids, esters or alcohols which melt when the ink is heated to jetting temperatures. Upon jetting, heated droplets impact the substrate and immediately freeze on the substrate surface. This phenomenon is advantageous in several respects in that dark, sharply defined print may be produced. This print may be slightly raised, suggesting that the print is engraved. Since the ink is solid at room temperature, during storage and shipment the colorant systems have less tendency to separate out of the ink. This has facilitated the use of various colorant systems, such as certain pigment-based systems, which would not normally have been used in liquid inks.
By way of example of the types of hot-melt inks employed to date, one may refer to U.S. Pat. No. 3,653,932 which discloses an ink containing a dialkyl ester of sebacic acid; U.S. Pat. No. 3,715,219 which discloses an ink containing a higher aliphatic alcohol; U.S. Pat. Nos. 4,390,369 and 4,484,948 which disclose inks containing a natural wax; EP 99,682 which discloses an ink containing stearic acid; and U.S. Pat. No. 4,659,383 which discloses an ink that contains an acid or alcohol of C20-C24 chain length, optionally in the presence of a ketone having a comparatively high melting point.
Not all of the aforementioned hot-melt inks exhibit high solubility of dyes, and thus the types of dyes that can be used are limited. In addition, these inks have not necessarily been stable against prolonged heating or repeated heat cycles. EP 181,198 discloses a hot-melt type ink that has a solid pigment dispersed in a wax having a melting point higher than 65.degree. C. or an aliphatic acid or alcohol of C18-C24 chain length. This ink, however, has the problem of poor dispersion stability.
Another disadvantage to many hot-melt phase- change inks is the fact that the physical properties of the printed images, such as resistance to abrasion and the like, are limited because the inks, when cooled, are only as hard as their principal components, which are typically resinous in nature. The inks must be liquid at operating temperatures, and because operating temperatures are naturally limited due to practical considerations, including the fact that the inks when applied to the substrate, such as paper, cannot cause burning or charring of the substrate, the melting temperature of the ink is likewise limited.
In U.S. Pat. No. 3,653,932 the composition is required to have a melting point which does not exceed 51.degree. C. and contains a didodecyl sebacate which is a highly viscous material. In order to overcome the problems associated with that formulation, U.S. Pat. No. 4,390,369 proposes the use of a composition which comprises a natural wax and has a melting point below about 75.degree. C. The use of natural waxes is also proposed in European application No. 097823, where the composition comprises a mixture of paraffin wax and stearic acid. However, such compositions do not adhere satisfactorily to plastics substrates and suffer from smudging. Additionally, problems are encountered due to the high viscosity of the components where synthetic materials are used.
U.S. Pat. No. 5,066,332 discloses a low-corrosion hot-melt ink that contains 0.5% to 10% by weight of a metallo-organic compound such as overbased calcium sulphonate, basic barium sulphonate and overbased sulphurized calcium alkyl phenate as a corrosion inhibitor.
U.S. Pat. No. 5,065,167 teaches an ink-jet ink including a waxy carrier that is solid at 25.degree. C. and liquid at the operating temperature of an ink-jet nozzle and a driver having a critical pressure greater than 10 atmospheres, the carrier and driver being miscible in liquid phase.
U.S. Pat. No. 5,053,079 relates to a dispersed, pigmented hot melt ink that contains a thermoplastic vehicle, a colored pigment, and a dispersing agent to inhibit settling or agglomeration of pigment when the ink is molten comprising an isocyanate-modified microcrystalline wax or lignite wax in an amount of 2 to 100 weight percent of the weight of the vehicle.
U.S. Pat. No. 5,047,084 relates to an ink-jet ink in the form of a microemulsion of an organic vehicle phase having a colorant dispersed therein and an aqueous phase containing a surfactant, the vehicle phase preferably being liquid at 70.degree. C. and solid at 20.degree. C.
U.S. Pat. No. 5,041,161 relates to jet inks that comprise vehicles, such as glyceryl esters, polyoxyethylene esters, waxes, fatty acids, and mixtures thereof, which are semi-solid at temperatures between 20.degree. and 45.degree. C.
U.S. Pat. No. 5,021,802 describes impulse ink or bubble-jet inks which comprise 90-99.9% by weight of aqueous sol-gel medium and 0.1-10% by weight colorant. The inks are thermally reversible sol-gels which are gels at ambient temperatures and sols at temperatures between about 40.degree.-100.degree. C.
U.S. Pat. No. 5,000,786 relates to an ink composition for use in an ink-on-demand, ink-jet printer which includes a first component, a second component having a higher melting point than the first component, and a colorant. During printing, the ink is melted and jetted onto a recording medium. The first component is selected to permeate from the melted dot of ink into the recording medium. As its concentration in the dot of ink decreases and the temperature of the ink cools, the second component with the colorant will solidify and the first component will fix the second component and colorant to the recording medium. The second component is indicated to be any of many, various compositions, including o-toluenesulfonamide and p-toluenesulfonamide. In all instances, a first component is present that typically is a wax-type of material, such as a petroleum wax or a candelilla wax having a high melting point but not more than about 70.degree. C., such as paraffin, paraffin wax, microcrystalline wax or solid-type wax formed from a combination of the compounds of these materials.
U.S. Pat. No. 4,931,095 relates to an ink for hot-melt ink-jet printing comprising a benzoate solvent which is solid at room temperature. This ink is suitable for jetting onto an opaque substrate such as paper for directly readable print, or onto a transparent substrate, such as an acetate or polycarbonate sheet, to make a projectable transparency.
U.S. Pat. No. 4,835,208 relates to a process for applying a thermoplastic composition as a series of discrete droplets from a non-contact ink jet printing apparatus to form separate drops on a substrate moving relative to the apparatus. The molten composition is thermally stable at the temperature of application and is applied at a temperature in excess of 100.degree. C. The process is said to be used to apply the molten composition to a variety of substrates using on-demand or continuous non-contact ink-jet application techniques and to be of especial use in the application of thermoplastic inks to non-porous substrates using an on-demand ink- jet printer.
U.S. Pat. No. 4,830,671 relates to a hot-melt ink composition consisting of a resinous binder which is prepared by the condensation reaction of about one stoichiometric equivalent of polymerized fatty acid, about two stoichiometric equivalents of a diamine and about two stoichiometric equivalents of a monocarboxylic acid and a colorant distributed throughout the resinous binder in an amount sufficient to impart a predetermined color to the resulting hot- melt ink composition.
U.S. Pat. No. 4,822,418 relates to an ink-jet composition for use in drop-on-demand ink jet printers comprising 85-99 weight percent of a vehicle consisting essentially of dibutyl sebacate and oleic acid. In the preferred embodiment, the dibutyl sebacate comprises between about 40 to 65, preferably about 60 weight percent of the ink. The subject ink, when utilizing an infrared readable colorant such as nigrosine base, is particularly suited for generating IR readable bar codes.
U.S. Pat. No. 4,758,276 relates to a stearic acid-containing ink-jet ink for use in an ink-jet apparatus which features good print quality. The ink-jet ink is discharged from the ink-jet ink apparatus at temperatures above ambient.
U.S. Pat. No. 4,741,930 relates to color printing characterized by high color density and color contrast, provided by jetting an ink composition composed of an optically clear, phase-change base material and a primary subtractive color dye which is soluble in the base material, onto the surface of a printing medium to form a multiplicity of closely spaced color dots or spots arranged to define a line or character on the medium. Each dot consists of one or more well defined, optically clear, semi-transparent color layers each of which has a different color so that the observed color of each dot is a true subtractive mixture of the colors in the layers. Specific ink compositions are also disclosed.
U.S. Pat. No. 4,684,956 relates to a process for applying a thermoplastic image-forming composition as a series of discrete droplets from a non-contact ink-jet printing apparatus to form separate drops on a substrate moving relative to the apparatus. The molten composition is thermally stable at the temperature of application and is applied at a temperature in excess of 100.degree. C.
U.S. Pat. No. 4,659,383 relates to a hot-melt impulse-ink-jet ink which comprises a C.sub.20 -C.sub.24 acid or alcohol-containing vehicle, preferably behenic acid, and a suitable coloring agent. The preferred ink has a melting point above about 65.degree. C., exhibits very good jettability, good heat stability at a reservoir temperature of 90.degree. C., good material compatibility, and an improved print quality. The preferred ink comprises about 80% natural behenic acid, 15% of a ketone (such as stearone), 3% coloring agent, and 2% of a plasticizer intended to improve print quality. The subject inks are jetted at relatively high temperatures (80.degree.-90.degree. C.) using impulse-ink-jet devices.
In related application Ser. No. 394,153, filed Jul. 1, 1982 (Lin et al), a number of hot melt impulse-ink-jet inks are disclosed which are described as being solid or semi-solid at ambient temperature. In addition to stearic acid, the Lin et al. application discloses that additives such as oleic acid, typophor black, nigrosine base, benzyl ether, compounded or chemically modified waxes (including natural or other synthetic substances), a coloring agent or dye, such as oil or solvent soluble dye, etc. may be used in formulating the disclosed hot-melt inks.
U.S. Pat. No. 5,006,170 relates to hot-melt ink compositions suitable for ink-jet printing which comprise a colorant, a binder, and a propellant. In one embodiment, the binder comprises a liquid crystalline material. The hot-melt inks with liquid crystalline binders are said to exhibit sharp melting points, which enables rapid melting of the ink and rapid solidifying of the ink on the printing substrate, thereby enabling rapid printing speeds. In addition, hot-melt inks with liquid crystalline binders are said to exhibit high shear thinning behavior, which means that under shear or stress, the melt viscosity is lowered. The inks are subjected to shear or stress as they pass through the jetting nozzle of the printer, and the ink viscosity is lowered during the printing process, which enables increased printing speed and also results in enhanced print quality, since the lowered viscosity of the ink enables a high degree of interaction between the ink and the printing substrate. Examples of suitable liquid crystalline materials for the inks include certain alkyl thio- beta -D-glucosides.
Other types of inks containing liquid crystalline materials include those of U.S. Pat. No. 3,776,742, which discloses an electrically conductive aqueous base ink for use in printing on a cellulose containing base member by formation of discrete droplets. The ink comprises a water soluble dye, a water soluble inorganic conductive material in an amount of 1 to 20 percent, a water soluble polyol in an amount of from 5 to 50 percent, an organic crystallizable material in an amount of from 5 to 20 percent, which organic material acts temporarily as a plasticizer for cellulose, and water. Typical crystallizable materials include sugars such as glucose, sucrose, fructose, and the like, including glucono D-lactone.
Inks containing aromatic sulfonamides have been suggested in the art. For example, U.S. Pat. No. 4,878,946 relates to a hot-melt type ink for thermal ink-jet printers that comprises an oil-soluble dye dissolved in at least one compound selected from the group consisting of a phosphoric acid ester, an aromatic sulfonamide, a hydroxybenzoic acid ester and a phthalic acid ester, the compound being in solid state at room temperature. The dye solvents that are suggested include phosphoric acid esters such as triphenyl phosphate, tri-p-tolyl phosphate, etc; aromatic sulfonamides such as N-cyclohexyl-p-toluene-sulfonamide, N,N-dichloro-p-toluenesulfonamide, N-butyl-p-toluenesulfonamide, benzenesulfonamide, p-toluenesulfonamide, o-toluenesulfonamide, benzylsulfonamide, etc; hydroxybenzoic acid esters such as ethyl p-hydroxybenzoate, n-propyl p-hydroxybenzoate, iso-propyl p-hydroxybenzoate, n-butyl p-hydroxybenzoate, iso-butyl p-hydroxybenzoate, n-heptyl p-hydroxybenzoate, n-nonyl p-hydroxybenzoate, phenyl salicylate, octyl salicylate, p-tert-butylphenyl salicylate, etc; and phthalic acid esters such as dicyclohexyl phthalate, diethyl phthalate, dodecyl phthalate, diphenyl phthalate, dimethyl iso-phthalate, dimethyl terephthalate, diethyl terephthalate. The only sulfonamide actually employed in a working example, however, is N-cyclohexyl-p-toluenesulfonamide.
U.S. Pat. No. 4,820,346 relates to a hot-melt ink for an ink-jet printer that is formed from a solid organic solvent having a melting point less than 150.degree. C., that may contain a sulfonamide, or mixture thereof, and a low molecular weight thermosetting resin having a high hydroxyl number, together with a dye. The ink is said to have excellent jetting characteristics, form a hard, stable print raised above the surface of the substrate, provide high dot resolution, and remain colorfast both in the ink jet printer and on the paper, even when exposed to continued high temperatures. The solid organic solvent, in a preferred embodiment, comprises one or more aromatic sulfonamides. In particular, primary alkyl (C.sub.1 -C.sub.9) benzenesulfonamides are said to have given excellent results. Particularly good results purportedly were obtained when the alkyl group was para to the sulfonamide group. One preferred solvent was a mixture of p-toluenesulfonamide and p-ethylbenzenesulfonamide. Preferably, the solvent mixture also comprised up to about 5 wt. % of a C.sub.3 -C.sub.9 alkylbenzenesulfonamide, with particularly good results supposedly being obtained using about 2 wt. % p-n-butylbenzenesulfonamide or about 2 wt. % p-n-nonylbenzenesulfonamide. Good results were also alleged to have been obtained using solvent mixtures of ortho- and para-toluenesulfonamide, such as that sold under the trademark "Santicizer 9" by the Monsanto Chemical Corporation. In all instances, the solid organic solvent was used in combination with a low molecular weight thermosetting resin having a high hydroxyl number.
Japanese published specification 55-54368 also discloses the use of certain low-crystallinity compositions for use in hot-melt ink-jet compositions. The utility of the crystalline materials disclosed therein is limited by the materials' lower melting points and other undesirable properties, such as excessive volatility and decomposition at operating temperatures. Another problem associated with such hot-melt inks is the tendency of the molten formulation to "super-cool" that is to continue to exist in the liquid stage at temperatures far below the melting point of the crystalline material. This is undesirable because it lengthens the time necessary for the printed images to sufficiently set to resist smearing, abrasion, and feathering.
Crystalline materials that would be capable of forming desirable, hard images, have not, in general, been used as components in hot-melt inks, because of their high melting points. If they have been used, it has only been in combination with a principal additional component having a lower melting point, such as a waxy material, thus causing such ink formulations to suffer from the same type of problems as the other compositions in the prior art that employ such soft, waxy, low melting components as the principal solvent.
Waxed-based hot-melt jet inks demonstrate good wettability but poor adhesion on many nonporous substrates, whereas crystalline hot-melt jet inks have poor wettability on low energy surfaces, but good adhesion to higher energy surfaces. Heretofore, no solution to this problem with crystalline-based hot melt jet inks has been found.
Ethylene glycol distearate has heretofore been used only in non-ink jet applications, such as those shown in Japanese Patent Nos. 2284971, 1242286, 1042456, 1026494, 62160284, or suggested for use in ink-jet formulations that are not of the hot-melt type, but rather are water-borne, as shown in Japanese Patent No. 3079682.